Increasing use is being made of paper substrates in the analysis and/or storage of biological materials. One such area of use concerns the growing need for rapid analysis of large quantities of blood samples in pharmacokinetic research, for example in drug discovery programmes. It is obviously desirable for such uses that the paper combines satisfactory mechanical properties with an ability to hold the biological material of interest in such a way that it can be subjected to analysis and/or further processing following storage. Examples of such papers are those known as FTA and FTA Elute (Whatman, part of GE Healthcare) described for example in U.S. Pat. Nos. 5,756,126 and 5,939,259. These papers have been impregnated with chemicals to provide sample preservation and to facilitate further processing of certain analytes, in particular nucleic acids.
However, in addition, it is necessary that, while held on the paper or after extraction, the analyte can be subjected to suitable analytical techniques, such as mass spectroscopy or immunoassays without interference in the process from any chemical constituents that may have been included in the coating on the paper. Thus, for example, the coatings used for suitable papers, such as those sold as FTA and FTA Elute, differ in composition and it is therefore necessary to bear in mind the nature of these chemicals and select carefully a suitable combination of substrate, analyte and analysis method. For samples such as blood, the analytes being detected, e.g. a drug being tested, may be present in very small quantities which are readily masked in the presence of certain coating chemicals. Further, many pharmaceutically important analytes, in particular proteins and other biomacromolecules are highly sensitive to denaturation, degradation and other disruptive events which lead to loss of recovery and/or loss of biological activity.
Chemicals introduced for stabilisation of nucleic acids do not necessarily provide stabilisation of proteins—in some cases they may even be detrimental to protein stability.
Cellulose fibres substituted with charged groups have been described in GB2016473A and WO2003016356. There is however no suggestion of using these fibres in preservation of biological samples and the substitution reactions are performed under conditions that cause substantial degradation of the fibres, making them unsuitable for industrial manufacture of paper. No data from continuous-web paper machines are presented.
It is therefore desirable to use substrates with sample-preserving treatments in which no chemicals are in a position to interfere with the analysis. Plain untreated cellulose papers such as the 903 or 31ETF papers (Whatman, part of GE Healthcare) are used for preservation of blood samples, but do not always give the desirable analytical recoveries and biological activities of e.g. sensitive proteins. Accordingly there is a need for sample preservation methods and sample preservation substrates with improved performance.